Typical fire protection systems of the kind employed in industrial and similar environments are required to provide for the substantially simultaneous release of a fire suppressant medium into a number of physically displaced portions of an area to be protected, either in automatic response to detection by any one or more sensors of the occurrence of a hazard condition associated with a fire, or in response to a manually executed command. For such purpose, containers of the suppressant medium under pressure are located in the various portions of the area and each is provided with a release valve having an electrically responsive initiator. Because of their fast initiation characteristics and ability to restrain high pressures prior to initiation, the release valves are usually of the rupture disc type having cartridge initiators adapted to fire and rupture the associated valve disc in response to energization of the cartridge with an electrical current having a magnitude in excess of a predetermined threshold value. The electrically responsive initiators each typically comprise a bridge wire element having special, preselected physical characteristics and disposed adjacent a small quantity of an explosive charge. The physical characteristics of the bridge wire are such that the explosive charge is ignited when an electrical current of at least the threshold value is passed through the bridge wire. Upon explosion of the explosive charge, a shock wave is delivered to the release valve which ruptures the disc thereof thereby initiating release of the fire suppressant medium.
In the past, the electrically responsive initiators were coupled in series relationship with each other and with a single source of electrical power for firing each of the initiators. In one sense, this arrangement was desirable since an electrical supervisory current of a magnitude less than the threshold or "firing" value could be passed through each of the initiators in order to monitor the electrical continuity of each of the bridge wires associated with the initiators; in this manner, the operative readiness of each of the initiators could be easily monitored. However, as a result of variations in the physical characteristics of the individual bridge wires associated with the initiators, some of the bridge wires fired more quickly than others resulting in a premature, open circuit in the series loop containing the initiators, thereby resulting in the firing of only a portion of the total number of initiators. More recently, in order to eliminate the aforementioned problem associated with firing the initiators when the latter are coupled in series relationship with each other, circuit means have been devised for normally coupling the initiators in series relationship with each other to allow monitoring the continuity of the bridge wires, but which switches (by means of relays or the like) the initiators into parallel relationship with each other immediately prior to the application thereto of the threshold, or "firing" level of electrical current, thereby assuring that the initiating current is delivered to each of the initiators, even if some of the bridge wires associated therewith are prematurely open circuited. This more recent arrangement for monitoring and firing the initiators was undesirable from the standpoint that additional circuitry including electromechanical components such as relays were required, thus making such systems not only more expensive from a manufacturing standpoint, but also less reliable due to the inclusion of additional circuitry and components. Also, as a further shortcoming of the series-monitor, parallel-fire arrangement for the initiators, it has been found that in some cases, rather than creating an open circuit upon firing, some bridge wires may produce a complete short circuit within an initiator when the threshold electrical current is passed therethrough; under these circumstances, a short circuited initiator located upstream toward the power source in the parallel circuit arrangement may divert the flow of firing current therethrough, thereby precluding the firing of actuators downstream from the short circuited initiator.
Known prior art fire protection systems, of the type described above, typically employ a single, isolated power supply source for providing the threshold level of electrical current to effect firing of the initiators. Such power supply is normally located in a control panel for example, at one end of a circuit configuration containing a plurality of parallel coupled initiators. In the event that the power supply lines connecting the plurality of initiators with the power supply source are short circuited due to a component malfunction or other catastrophy, the resulting short circuit prevents the delivery of the threshold value of electrical current to the initiators thereby disabling the system since the initiators are prevented from firing.
From the foregoing, it is clear that there is a need in the art for a fire protection system which allows continuous monitoring of each of the bridge wires respectively associated with a plurality of parallel coupled, electrically responsive initiators, but whose operation is not thwarted as a result of an unintentional shorting of the circuit which electrically intercouples the initiators for firing.